What is a skin test for allergy?

This is a test done on the skin to identify the allergy substance (the allergen) that is the trigger for an allergic reaction.

How is an allergy skin test done?

A small amount of the suspected allergy-provoking substance (the allergen) is placed on the skin. The skin is then gently scratched through the small drop with a special sterile needle. An allergy skin test is also called a scratch test.

What is a positive skin test?

If the skin reddens and, more importantly, if it swells, then the test is read as positive and allergy to that substance is considered probable.

Can you give an example of a skin test?

If a specific food allergy is suspected, a skin test uses a dilute extract of the suspect food. A small drop of this particular liquid extract is placed on the skin of the forearm or back. This underlying skin is gently scratched through the small drop with a special sterile needle. If the skin reddens and, more importantly, if it swells, then the test is read as positive. If there is no reaction, it is read as negative.

If the skin test is positive, it implies that the patient has a type of antibody (IgE antibody) on specialized cells in the skin that release histamine to cause redness and itching. (These cells are called mast cells and the IgE antibody bound to them is specific to the food being tested.)

What are the advantages of skin tests?

Skin tests are rapid, simple, and relatively safe. They can be very helpful in specifically identifying causes of allergies.

Is there danger to a skin test?

In some extremely allergic patients who have severe reactions called anaphylactic reactions, skin testing cannot be used because it could evoke a dangerous reaction. Skin testing also cannot be done on patients with extensive eczema.

What is done if a skin test can't be done?

For these patients a doctor may use special blood tests, such as the RAST and the ELISA. These tests measure the presence of specific types of IgE in the blood.

These tests may cost more than skin tests, and results are not available immediately. As with skin testing, positive RAST and ELISA tests do not by themselves necessarily make the final diagnosis.

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